Pebble heater apparatus and method for heat exchange



Feb. 3, 1953 s. P. ROBINSON PEBBLE HEATER APPARATUS AND METHOD FOR HEAT EXCHANGE Filed Jan. 2, 1948 AIR I 24\ Hc WATER PRODUCT REACTANTS l2 3 Sheets-Sheet l |s max 10R.

s. P. ROBINSON ATTORNEYS Feb. 3, 1953 S. P. ROBINSON PEBBLE HEATER APPARATUS AND METHOD FOR HEAT EXCHANGE Filed Jan. 2, 1948 WATER STEAM V [0 PRODUCT FEED FIG. 2

3 Sheets-Sheet 2 TO FLUE OR HEAT RECOVERY IN VEN TOR. S. P. ROBINSON ATTORNEYS 3, 1953 s. P. ROBINSON 2,627,497

PEBBLE HEATER APPARATUS AND METHOD FOR HEAT EXCHANGE Filed Jan. 2, 1948 3 Sheets-Sheet 3 10000 20000 30000 40000 50000 60000 70000 80000 90000 NUMBER OF CYCLES EFFECT OF TEMPERATURE ON PEBBLE LIFE FIG 3 l I l I l l I I O O O O O O 0) w h D n if SI-FIGEBd NHMOHHNO B'DVLNEDHHE! IN V EN TOR. -S. P. ROBINSON wwzw A 7' TORNE'YS UNITED STATES PATENT OFFICE PEBBLE HEATER APPARATUS AND METHOD FOR HEAT EXCHANGE Sam P. Robinson, Bartlesville, kla., assignor to Phillips Petroleum Company, a corporation of Delaware Application January 2, 1948, Serial No. 186

13 Claims. 1

This invention relates to an improved process for the continuous transfer of heat from a heat ing zone to a heat absorption zone. In one specific aspect it relates to an improved process for conducting endothermic chemical reactions at high temperatures. In another specific aspect it relates to an improved process for conducting hydrocarbon conversions at elevated temperatures. In still another aspect it relates to an imor counteract these internal stresses, there occurs within the pebble a rapid increase in crystal size. As the crystals increase in size, many small crystals are absorbed by the larger reinforcing crystals with a resulting shattering of many crystal to crystal face bonds and lowered resistance to physical shock. Finally, the pebbles are broken, either from the thermal shock or physical shock in handling, or both.

proved pebble heater. I propose to remedy these faults by providing In the pebble heaters of the prior art, various a pebble tempering or soaking zone between the arrangements are employed for recovering the usual heating zone and the absorption zone. The heat from the reaction products and the pebbles pebbles flow downward through the heating zone after they have passed through the reaction zone, into the temperating zone and thence downward but essentially these heaters comprise a pebble therethrough into the reaction zone. Thus, the heating zone directly above a reaction zone. Hot Pebbles are contacted by the hottest gases imcombustion gases flow into the lower portion of mediately on flowing into the tempering zone. the heating zone and contact the pebbles just The hot combustion or heating gases are introbefore the pebbles flow through a constricted duced into the upper portion of the tempering throat into the reaction zone. In these heaters Zone and are ca to fi W d w w through the pebbles are introduced into the top of the the tempering chamber concurrently with the heating zone and are allowed to flow downward p bb es and a o e eu h a gas Outlet in through this zone by gravity in direct contact the lower portion of said tempering zone. with the upward flowing heating gases. There These gases are moved through a conduit to a is no method for producing a completely uniform gas inlet in the lower portion of said heating zone flow of pebbles through such a heating zone and and are caused to flow upward through said heatsome pebbles flow through the heating zone coning zone countercurrently to the pebbles to be siderably faster than others. Since the distriburemoved through a Vent in the upp r p t on Of tion of the up-flowing heating gases is fairly even said heating zone. By this concurrent flow, th throughout the heating zone, those pebbles hav- 3Q heating gases and the pebbles approximate a ing the longer residence time in this zone are state of thermal equilibrium for a considerable heated to a much higher temperature than those time before the pebbles leave the tempering zone, which pass through rapidly. It is easily possible thereby producing a more uniform temperature that the te p y between individual throughout the entire fluent mass of pebbles and pebbles as much as 500 F. relieving internal stresses within the pebbles be- In a hydrocarbon Conversion P an Optlfore said pebbles are quenched in the reaction mum cracking temperature is selected but in the Zone heaters presently in use, some of thepebbles will The principal object of my invention is to be hotter and some 9 than thls optlmumvide an improved method of transferring heat The hydrocarbons which contact the cool pebbles from a heating Zone to an absorption zone. ZSiiZZ$tZ?SEES1ZEtEfZ3232323531232 tfil Another Object is to provide an v d continuous process for conducting endothermic results in a low yield of desired products, and in addition, the overcracking tends to coke up the gfi ai at h 1gh e ures pebbles and the gas outlet. Unfortunately, there g er 0 ject 15 to P de a p ved isno chemical equilibrium between overcracked P6 9 ea and undercracked gases to equalize the efiect of Numerous other objects and advantages will be hot and cold pebb1eS pp rent to those skilled in the art from reading Another undesirable result of uneven heating the following spefification, s, and the acof pebbles is excessive pebble breakage or spalling companylng drawlngs. from thermal shock. These refractory pebbles, the w ngs: which are used in high temperature systems, are Fljglll'e 1 is an elevatienal View, With parts conheated to high temperatures and then suddenly ventlenally shown and parts broken away, of a cooled. This produces severe internal stresses pebble furnace embodying the present inve ti within the pebble. In an effort to compensate for Figure 2 is a modification of 1 showing the preheating and tempering zones combined in a single chamber.

Figure 3 is a graphic representation of pebble life at various temperatures.

In Figure 1, number 5 is the conversion or heat absorption chamber showing the refractory lining 6, insulating wall 6a, and outer wall 6b, and pebbles I. The reactant materials flow through conduit 8 into the reaction chamber and upward through the downward moving pebbles, and the reaction products are removed through conduit 9 to an external water quenching zone It, and from there to a product recovery system (not shown). The flow of pebbles I I from the reaction chamber through pebble outlet I2 is controlled by a rotary valve [3. These pebbles flow through the pebble conduit 14 into the inlet l5 of a pebble conveyor I6, here shown as a bucket type conveyor. The pebbles are lifted by this conveyor and discharged through outlet I! into pebble conduit 18 through which they move into pebble heating chamber l9.

As the pebbles flow downward through the heating chamber, they are met by the upward flowing heated gases introduced into a lower portion at gas inlet 20 of said heating chamber through conduit 2i. These gases are removed from an upper portion of heating chamber 19 through an exhaust conduit 22. The pebbles flow from the heating chamber through a constricted throat 23 into the tempering chamber 2 5. Air and a combustible material, such as hydrocarbons, are burned in an external combustion chamber 26 and the hot gases are introduced into the upper portion of the tempering chamber at gas inlet 25 through conduit 2'5, controlled by a valve 28. If desired, a portion of the hot gases may be introduced directly into the heating chamber I9 atgas inlet 30 through conduit 29 controlled by valve 3!. The hot gases flow downward through the tempering chamber concurrently with the moving bed of pebbles and are removed at gas outlet 32.

The tempering chamber and the reaction chamber are in communication with each other .by means of a constricted throat 33 similar to the one between the heating and the tempering chambers. To prevent the loss of reaction prodnote by their flowing from the reaction chamber into the tempering chamber and also to prevent the contamination of the reaction products by combustion gases flowing from the tempering chamber into the reaction chamber, a stream of steam is introduced through line 3 2 into constricted throat 33, thus blocking the flow of gases in both directions.

In Figure 2, heating gases are admitted into the combined heating and tempering chamber 36 through a gas conduit 3i which discharges into the tempering zone 38 of the combination chamber at a point intermediate the middle and the bottom. The downward flowing stream of gases is removed near the bottom of the tempering zone through gas conduit 32, which branches downstream, one stream 39 flowing into the heating zone through variable delivery gas pump or blower 40, the other stream flowing through branch 4!, controlled by throttle valve 42 to join the exhaust line 22 of the heating zone @3 at a point downstream of throttle valve M.

Figure 3 is a graphic representation of the life of a high grade of alumina pebbles at various pebble temperatures. The range of 1700 F. to 2500 F. represents the temperatures to which -pebbles are heated in usual pebble furnace operations. These pebbles have a melting point of about 3700 F. Above 2000 F., they begin to lose rigidity and become subject to plastic deformation when under intense strain. However, these pebbles do not exhibit crystal growth on continuous heating at 3000" F. for 25 hours. It is therefore evident that the breakage of pebbles is not attributable to heatingalone, but is the result of internal stresses produced by heating and cooling without any provision for stress relieving.

After an examination of these curves, the disadvantages of countercurrent heating of pebbles without any tempering are readily apparent.

Operations My inventicn is adapted to a wide variety of processes wherein heat must be supplied to an absorption or reaction zone for conducting endothermic chemical reaction at high process temperatures, e. g. above 1000 F. It is particularly adapted to thermal cracking of light hydrocarbons. This heat transfer is accomplished by heating a mass of refractory pebbles in a heating zone and allowing them to flow in a continuous stream through the reaction zone where the pebbles give up part of their sensible heat to the reaction and are then transferred back to the heating zone for recycling through the system.

The pebbles referred to herein preferably are of a refractory material and of such shape that they will flow readily through the chambers or zones. However metal alloys such as high chrome, chrome-nickel or chrcme-moly steels may be used at the lower temperature levels. However, for high temperature levels, the pebbles are preferably of a more highly refractory material such as alumina, zirconia, or mullite. They are preferably spherical in form and may range in size from A; inch to 1 inch, but sizes of 4 inch to inch are preferable.

I make use of three chambers or zones, preferably in substantially vertical arrangement, through which pebbles pass. The lower chamber 5 is a conversion or absorption chamber where the pebbles i are contacted with the reactant or absorption materials. Above this conversion chamber and in communication therewith by means of a constricted throat 33 is a chamber 25 which I have chosen to call a tempering chamber, and above the tempering chamber and in communication therewith by means of a constricted throat 23, is a pebbl heating chamber I9. 5

As the pebbles flow from the heating chamber into the tempering chamber, they are contacted by hot gases, usually combustion gases, from an outside source, which gases enter the upper portion of the tempering chamber through conduit 27. These gases are caused to flow downward through the tempering chamber concurrently with the heated pebbles and are removed at or near the bottom of the tempering chamber through an external conduit 2i which leads to the lower portion of the heating chamber l9 above. The gases move upward through the heating chamber countercurrently with the pebbles and are removed through an exhaust vent 22 in the top of the heating chamber.

This function of the pebble heating chamber is the same as that of other pebble heaters where the pebbles are heated by direct contact with a countercurrent stream of hot gases. The length of th constricted throat between the heating and tempering chambers and the location of the gas inlet 25 and outlet 32 in the tempering chamber and the gas inlet 35 in the heating chamber are ber.

In a heat transfer system wherein efficient removal of heat from the heating gases is of greater importance than uniformity of pebble temperatures, countercurrent contact between heating gases and pebbles would be advisable. However, if uniformity of pebble temperatures is of greater importance, concurrent contact would be better. I achieve both the uniformityof pebble temperatures and efficient heat removal in my invention. In a countercurrent system, the temperature of the absorbing material is low on entering the heating chamber. The increase in temperature of the pebbles is slow at first and as the pebbles approach the point at which the gases are introduced the temperature increase accelerates rapidly and the rate of increase is greatest at the point where the heating gases are introduced. The spread between the pebble temperature and the heating gas temperature is of a high order of magnitude.

In a concurrent heating system, the temperature rise of the pebbles is extremely rapid when the pebbles and heating gases first establish contact, with the result that during the latter portion of the residence time of the pebbles in the tempering chamber, the gases and pebbles are substantially in thermal equilibrium. The final spread between gas temperature and pebble temperature is very narrow. The result of this is a substantially uniform temperature throughout the fluent mass of pebbles and a substantially uniform temperature throughout the mass of each individual pebble.

It may in some cases, be advantageous to supply only part of the fresh heating gases to the tempering chamber and at the same time admit a part of the heating gases to the preheating zone. By having adjustable controls 23 and 3i on the fresh gas supply to each chamber, a wide range of flexibility is possible for meeting various heating and temperature demands.

The middle tempering chamber 24 is the most important feature of my invention. As related above, the pebbles are contacted by the heated gases immediately on flowing into'the tempering chamber and these gases flow through the tempering chamber concurrently with the pebbles.

Two important results are achieved by the concurrent flow. 'In the first place, temperature variations within the fluent mass are equalized so that the temperature throughout approximates the optimum or predetermined temperature.

usual process where there is no stress-relieving.

In a modification of my invention, the pebble heating and tempering zones may be combined in a single unit 36. In this case, the heating gases are introduced into this combination heating and tempering chamber at an intermediate point and are caused to flow upward and downward from the inlet 30 in countercurrent and concurrent contact with the downward flowing pebbles.

The downward flowing gases are removed near the bottom of tempering zone through conduit i is valuable in any pebble heater.

32 andall or a part of them are introduced into the pebble heating zone at a point upstream of the heating gas inlet 30 withrespect to the flow of pebbles. The combined upward flowing stream of gases is removed near the top of the heating zone through conduit 22. The spent heating gases may be transferred to a heat exchangerfor further recovery of the heat.

My invention will be more fully appreciated by reference to the following examples, which have been selected to show, in a minimum of space, the advantages to be gained through my invention.

Example I The following is an example of the efiect of uneven pebble temperature in a hydrocarbon cracking system for the production of ethylene. A feed stock of mol per cent of propane and 25 mol per cent of ethane is to be cracked at a temperature of 1600 F. for the production of ethylene. Using reaction velocity constants and product distribution analyses as reported by Schutt in Chemical Engineering Progress, March 1947, pp. 103-116, a reaction time of 0.26 second will allow cracking of the propane and 73% cracking of the ethane. The product gas distribution under these conditions is shown in the first column of Table I.

The actual pebble temperatures vary from 1500 F. to 1700" F., a conservative range in thepresent type pebble heaters, and 25% of the total feed is cracked at 1500 F., 25% at 1700 F., and only 50% at the design temperature of 1600 F. The gases and amounts produced would be as shown in the last four columns of Table I.

TABLE I Feed stock, 25 mol per cent C H +75 mol per cent CeH Design Temperature, 1600 F.

Reaction Time, 0.26 second Total Feed, mols Uni- T (Comform hon-uniform posite) Temperature F 1, 600 1.500 1, 600 1, 700 Degree of CaHg cracking,

percent 95 73 95 100 Degree of CzHa cracking,

percent 73 35 73 93 Reaction Products Mols produc gas/100 M015 0 Feed H; 45.0 9 22. 5 12. 7 41.1 CH4 64. 3 9. 2 32. 2 l9. 9 61. 3 C2H4E. 48. 5 9. 0 24. 2 9. 7 42. 9 C254; 22. 5 6. 2 11. 3 5. 2 22. 7 03116.- 3. 5 3. 2 1.7 0. 2 5. 1 CaH 4. 3 5.1 2.1 0.2 7. 4 04+ 3. 2 0.2 1.0 1.1. 2. 9

Total 191. 3 183. 4

*Includes 02131.

mols of feed is increased by 13%.

This is a single example of the effect of nonuniform heating of pebbles. It is probable that there would be an even greater spread in actual pebble temperatures with present type heaters, and the disadvantages would be even more pronounced.

While a process has been described wherein my invention is useful, it is to be understood that its use is not limited to any specific process but My invention is not limited by any specific examples, but is limited only by the following claims,

Having described my invention, I claim:

1. In a continuous process for the transfer of heat from a heating zone to an absorption zone which comprises passing a stream of solid heat transfer elements through a series of zones comprising at least a preheating .zone, a tempering zone and an absorption zone, the improvement which comprises passing the elements through said preheating zone in direct contact with a countercurrent stream of hot gases; passing the preheated elements through said tempering zone in the absence of combustion therein and in direct contact with a concurrent stream of hot gases of sufiicient temperature and quantity and .for a sufficient period of time to temper said elements and approximate a state of thermal equilibrium between hot gases and elements; passing the elements from said tempering zone into and through said absorption zone whereby heat is supplied to said absorption zone; and returning the elements to said preheating zone for recycling.

2. In a continuous process for the transfer of heat from a heating zone to an absorption zone which comprises passing a stream of solid heat transfer elements downward through a series of zones comprising at least .a preheating zone, a tempering zone and an absorption zone, the improvement which comprises passing the elements downward through said preheating zone in direct contact with a counter current stream of eiiluent hot gases from said tempering zone; passing the preheated elements downward through said tempering zone in direct contact with a concurrent stream of hot combustion gases which are hottest at the top of said zone and of sufficient temperature and quantity and for a sumcient period of time to temper said elements and approximate a state of thermal equilibrium between hot gases and elements; passing the elements from said tempering zone downward into and through said absorption zone whereby heat is supplied to said absorption zone; and returning the elements to said preheating zone for recycling.

3. In a continuous process for the transfer of heat from a heating zone to an absorption zone which comprises passing a stream of solid heat transfer elements downward through a series of zones comprising at least a preheating zone, a tempering zone and an absorption zone, the improvement which comprises passing the elements downward through said preheating zone in direct contact with a countercurrent stream of mixed gases composed of the hot effluent gases from the tempering zone and additional fresh hotter gases; passing the preheated elements downward through said tempering zone in direct contact with a concurrent stream of hot combustion gases which are hottest at the top-of said zone and of suflicient temperature and quantity and for a sufficient period of time to temper said elements and approximate a state of thermal equilibrium between hot gases and elements; passing the elements from said tempering zone downward into and through said absorption zone whereby heat is supplied to said absorption zone; and returning the elements to said preheating zone for recycling.

4. In a continuous process for the conversion of hydrocarbons at elevated temperatures which comprises passing a stream of solid heat transfer elements downward through a series of chambers comprising at least an element preheating chamber, an element tempering chamber, and a conversion chamber; passing a stream of hot combustion gases through the tempering and preheat- '8 ing chambers in direct contact with the elements thereby heating them substantially above the predetermined conversion temperature, contacting said elements in the conversion chamber with a stream of hydrocarbons under conditions resulting in the desired conversion and passing the elements from the bottom of the conversion chamber back to the top of the preheating chamber for recycling through the system; the improvement comprising passing the elements first downward through the preheating chamber in direct contact with a countercurrently moving stream of eiiiuent hot gases from the tempering chamber, and then passing the hot elements from the preheating chamber through the tempering chamber in direct contact with a concurrently moving stream of hot combustion gases which are at sufficient initial temperature to heat the elements to the predetermined temperature substantially immediately, and then maintaining the hot elements in contact with the hot combustion gases during the time of stay in the tempering chamber whereby a substantially uniform temperature of the elements entering the conversion chamber is produced.

5. In a continuous process for the conversion of hydrocarbons at a temperature of 1100 F. to 3500" F. which comprises passing a stream of heat transfer pebbles downward through a series of substantially vertically arranged chambers, comprising at least an upper pebble preheating chamber, a middle pebble tempering chamber, and a lower conversion chamber; passing a stream of hot combustion gases through the tempering and preheating zones in direct contact with the pebbles thereby heating them substantially above the predetermined conversion temperature, contacting said pebbles in the conversion zone with a stream of hydrocarbons under conditions resulting in the desired conversion and passing the pebbles from the bottom of the conversion chamber back to the top of the preheating chamber for recycling through the system; the improvement which comprises passing the recycled pebbles first downward through the pr.- heating chamber in direct contact with a countercurrently moving stream of efiluent hot gases from the tempering chamber, and then passing the hot pebbles from the preheating chamber through the tempering chamber in direct contact with a concurrently moving stream of hot combustion gases which are at sufiicient initial temperature to heat the pebbles to the predetermined temperature substantially immediately, and then maintaining the hot pebbles in contact with the hot combustion gases during the time of stay in the tempering chamber whereby a substantially uniform temperature throughout the mass of recycled elements is produced.

6. In a continuous process for conducting endothermic chemical reactions at elevated temperatures which comprises passing a stream of pebbles downward through a series of substantially vertically arranged chambers, comprising at least an upper pebble preheating chamber, a

middle pebble tempering chamber, and a lower reaction chamber; passing a stream of hot combustion gases through said tempering and preheating chambers, whereby the pebbles are heated substantially above the predetermined reaction temperature, contacting said pebbles in the conversion chamber with a stream of reactants under conditions resulting in the desired reaction and passing said pebbles from the bottom of the reaction chamber back to the top of the preheating ,9 chamber for recycling through the system; the improvement which comprises passing the pebbles first downward through the preheating chamber in direct contact with a countercurrently moving stream of effluent hot gases from the temper- -ing chamber, and then passing the hot pebbles from the preheating chamber through the tempering chamber in direct contact with a concurrently moving stream of hot combustion gases which are at suflicient initial temperature to heat said pebbles to the predetermined temperatures substantially immediately, and then maintaining said pebbles in contact with the hot combustion gases during the time of stay in said tempering chamber thereby producing a substantially uniform temperature throughout the mass of said pebbles.

'7. In a continuous process for conducting endothermic chemical reactions at elevated temperatures which comprises passing a stream of pebbles downward through a series of substantially vertically arranged chambers, comprising at least an upper pebble preheating chamber, a middle pebble tempering chamber, and a lower reaction chamber; passing a stream of hot combustion gases through said tempering and preheating chambers, whereby the pebbles are heated substantially above the predetermined reaction temperature, contacting said pebbles in the conversion chamber with a stream of reactants under conditions resulting in the desired reaction and passing said pebbles from the bottom of the reaction chamber back to the top of the preheating chamber for recycling through the system; the improvement which comprises passing the pebbles first downward through the preheating chamber in direct contact with a'countercurrently moving stream of mixed gases composed of the hot efiluent gases from the tempering zone and additional fresh hot gases; and then passing the hot pebbles from the preheating chamber through the tempering chamber in direct contact with a concurrently moving stream of hot combustion gases which are at sufficient initial temperature to heat said pebbles to the predetermined temperature substantially immediately, and then maintaining said pebbles in contact with the hot combustion gases during the time of stay in said tempering chamber thereby producing a substantially uniform temperature throughout the mass of said pebbles.

8. A fluid heater for heating fluids with solid heat transfer pebbles, comprising in combination, a heat absorption zone, a pebble tempering zone above and in communication therewith, a pebble preheating zone above said tempering zone and in communication therewith, a combustion chamber external to said tempering zone, means to supply heating gases from said combustion chamber to an upper portion of said tempering zone and a lower portion of said preheating zone, gas conduit means other than the last-mentioned means connecting a lower portion of said tempering zone with a lower portion of said preheating zone, an exhaust gas conduit connected to an upper portion of said preheating zone, a pebble inlet in an upper portion of said preheating zone, said heat absorption zone having a fluid inlet in a lower portion and a fluid outlet in an upper portion, a pebble outlet in a lower portion of said absorption zone, conveyer means for transferring heat transfer pebbles from pebble outlet in said absorption zone to pebble inlet in said preheating zone.

9. A fluid heater for heating fluids with solid heat transfer pebbles, comprising in combination a heat absorption chamber, a tempering chamber above and in communication with said absorption chamber, a preheating chamber above said tempering chamber and in communication therewith, a pebble elevator having an inlet below and in communication with a lower portion of said absorption chamber and a discharge above and in communication with the upper portion of said preheating chamber, a combustion chamber having an inlet and an outlet, means to supply fuel and air to the inlet of said combustion chamber, a branched conduit connecting the outlet of said combustion chamber with an upper portion of said tempering chamber and with a lower portion of said preheating chamber, a gas conduit connecting a lower portion of said tempering chamber with a lower portion of said preheating chamber, said absorption chamber having a fluid inlet anda fluid outlet, means to supply fluid to be heated to said fluid inlet'and conduit means receiving heated fluid from said fluid outlet, quenching means in said conduit means, and an exhaust conduit connected to an upper portion of said preheating chamber. 1

10. A fluid heater for heating fluids with solid heat transfer pebbles comprising in combination a conversion chamber, a tempering chamber above and in communication with said conversion chamber, 'a preheating chamber above and in communication with said tempering chamber, a pebble elevator having an inlet below and in communication with said conversion chamber and a discharge above and in communication with said preheating chamber, a combustion chamber having an inlet and an outlet, means to supply fuel and air to the inlet of said combustion chamber, a conduit connecting the outlet of said combustion chamber with an upper portion of said tempering chamber, a conduit connecting a lower portion of said tem ering chamber with a lower portion of said preheating chamber, said conversion chamber having a fluid inlet and a fluid outlet, means to supply fluid to be heated to said fluid inlet and conduit means receiving heated fluid from said fluid outlet, and an exhaust conduit connected to an upper portion of said preheating chamber.

11. A fluid heater for heating fluids with pebbles comprising in combination a conversion chamber, a tempering chamber above and in communication with said conversion chamber, a preheating chamber above and in communication with said tempering chamber, a pebble elevator having an inlet below and in communication with said conversion chamber and a discharge above and in communication with said preheating chamber, a combustion chamber having an inlet and an outlet, means to supply fuel and air to the inlet of said combustion chamber, a conduit connecting the outlet of said combustion chamber with an upper portion of said tempering chamber, a conduit connecting a lower portion of said tempering chamber with a lower portion of said preheating chamber, and an exhaust conduit connected to an upper portion of said preheating chamber.

12. A fluid heater for heating fluids with solid heat transfer pebbles comprising a heat absorption chamber and a single pebble preheating and tempering chamber, said absorption chamber having a fluid inlet below and a fluid outlet above, a pebble outlet in a lower portion and a pebble inlet in an upper portion of said absorption chamber, said preheating and tempering chamber having a pebble inlet in an upper portion and a pebble outlet in a lower portion, conveyer means for transferring pebbles from the pebble outlet of said absorption chamber to the pebble inlet of said preheating and tempering chamber, pebble conduit means connecting the pebble outlet of said preheating and tempering chamber with the pebble inlet of said absorption chamber, a heating gas inlet in'said preheating and tempering chamber in a lower portion of said chamber, means for supplying heating gases thereto, an exhaust gas conduit leading from an upper portion of said preheating and tempering chamber, a branched gas conduit leading from a point of said preheating and tempering chamber lower than said heating gas inlet, one branch of said branched conduit leading to and joining the exhaust gas conduit, the second branch of said branched conduit reentering said preheating and tempering chamber at a point upstream of said heating gas inlet with respect to flow of pebbles and below said exhaustgas conduit, and a blower in :said second branch.

13; A fluid heater for heating fluids with pebbles comprising in combination a heat absorption chamber, a pebble tempering chamber thereabove, a pebble preheating chamber above said tempering chamber, pebble throats of much smaller cross-section than said chambers inter- 1'2 connecting same fordownward flow of pebbles therethrough and for preventing substantial flow of gases through said throats from one chamber to another, conveyor means for transferring pebbles from a lower portion of said heat absorption chamber to an upper portion of said preheating chamber, a combustion chamber for generating hot gases, a conduit connecting the outlet of said combustion chamber with an upper portion of said tempering chamber for introducing said hot gases thereto, a conduit connected to a lower portion of said tempering chamber for withdrawing gases therefrom, means to supply heating gases to a lower portion of said preheating chamber, and an exhaust conduit connected to an upper portion of said preheating chamber.

SAM P. ROBINSON.

REFERENCES CITED- The following references are of record in the file of this patent:

UNITED S'IATESv PATENTS Number Name Date 2,389,636 Ramseyer Nov. 2'7, 1945 2,398,954 Odell Apr. 23, 1946 2,416,214 Payne Feb. 18, 1947 2,445,092 Utterback July 13, 1948 

1. IN A CONTINUOUS PROCESS FOR THETRANSFER OF HEAT FROM A HEATING ZONE TO AN ABSORPTION ZONE WHICH COMPRISES PASSING A STREAM OF SOLID HEAT TRANSFER ELEMENTS THROUGH A SERIES OF ZONES COMPRISING AT LEAST A PREHEATING ZONE, A TEMPERING ZONE AND AN ABSORPTION ZONE, THE IMPROVEMENT WHICH COMPRISES PASSING THE ELEMENTS THROUGH SAID PREHEATING ZONE IN DIRECT CONTACT WITH A COUNTERCURRENT STREAM OF HOT GASES; PASSING THE PREHEATED ELEMENTS THROUGH SAID TEMPERING ZONE IN THE ABSENCE OF COMBUSTION THEREIN AND IN DIRECT CONTACT WITH A CONCURRENT STREAM OF HOT GASES OF SUFFICIENT TEMPERATURE AND QUANTITY AND FOR A SUFFICIENT PERIOD OF TIME TO TEMPER SAID ELEMENTS AND APPROXIMATE A STATE OF THERMAL EQUILIBRIUM BETWEEN HOT GASES AND ELEMENTS; PASSING THE ELEMENTS FROM SAID TEMPERING ZONE INTO AND THROUGH SAID ABSORPTION ZONE WHEREBY HEAT IS SUPPLIED TO SAID ABSORPTION ZONE; AND RETURNING 